1. Field of the Invention
The present invention relates to a method, device, and system for waveform shaping of signal light.
2. Description of the Related Art
A Mach-Zehnder interferometer (MZI) type optical gate is known as a conventional waveform shaping device for performing waveform shaping on the optical level. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is output from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs-SOA having opposite end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 xcexcm band, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate.
A nonlinear optical loop mirror (NOLM) is known as another conventional waveform shaping device. The NOLM includes a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as an SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
In an optical fiber communication system that has been put to practical use in recent years, a reduction in signal power due to transmission line loss, coupling loss, etc. is compensated by using an optical amplifier such as an erbium doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier, which functions to linearly amplify a signal. In this kind of optical amplifier, amplified spontaneous emission (ASE) noise generated in association with the amplification is added to cause a reduction in signal-to-noise ratio (S/N ratio), so that the number of repeaters is limited to result in the limit of a transmission distance. Further, waveform degradation due to the chromatic dispersion owned by an optical fiber and the nonlinear optical effects in the fiber is another cause of the transmission limit. To break down such a limit, a regenerative repeater for digitally processing a signal is required, and it is desirable to realize such a regenerative repeater. In particular, an all-optical regenerative repeater capable of performing all kinds of signal processing in optical level is important in realizing a transparent operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all-optical regenerative repeater are amplitude restoration or reamplification, waveform shaping or reshaping, and timing restoration or retiming. These functions are referred to as 3R functions, and in particular, the first and second functions are referred to as 2R functions.
The 2R functions can be provided by combining a waveform shaping device and an optical amplifier, or by using a waveform shaping device having an optical amplifying function. Further, the 3R functions can be provided by additionally using a clock recovery circuit in parallel to the 2R functions.
The present inventor has already proposed a waveform shaping device for providing the 2R functions and/or the 3R functions (Japanese Patent Application No. Hei 11-293189). In this device, two NOLMs are combined to thereby increase the degree of wavelength conversion in the case of obtaining a function of waveform shaping or optical gate.
Particularly in the waveform shaping device for providing the 3R functions, there is a case that sufficient 3R functions cannot be obtained according to the degree of deterioration of an optical signal on which the extraction of a clock pulse in the clock recovery circuit is based.
Further, in the case of applying wavelength division multiplexing (WDM) to dramatically increase a transmission capacity, it is expected that the waveform shaping device for providing the 3R functions may be complicated according to the number of WDM channels. Accordingly, a waveform shaping device suitable for WDM is demanded.
It is therefore an object of the present invention to provide a method, device, and system for waveform shaping which can obtain sufficient 3R functions.
It is another object of the present invention to provide a method, device, and system for waveform shaping which are suitable for WDM.
Other objects of the present invention will become apparent from the following description.
In accordance with an aspect of the present invention, there is provided a method comprising the steps of supplying signal light to a first waveform shaper to obtain intermediate signal light; dividing said intermediate signal light into first and second signal lights; supplying said first signal light to a clock recovery circuit to obtain a clock pulse; and supplying said second signal light and said clock pulse to a second waveform shaper to obtain regenerated signal light synchronous with said clock pulse.
According to this method, the clock pulse can be obtained according to the intermediate signal light obtained in the first wavelength shaper. Accordingly, the fidelity in clock regeneration can be improved to obtain sufficient 3R functions. Further, this method can be effectively applied to WDM as will be hereinafter described.
Preferably, each of said first and second waveform shapers comprises an optical gate using switching by cross-phase modulation. For example, a nonlinear optical loop mirror (NOLM) may be used as the optical gate. The NOLM comprises a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path formed of a nonlinear optical medium for connecting said first and second optical paths, and a second optical coupler including a third optical path directionally coupled to said loop optical path.
Preferably, said clock recovery circuit comprises a mode-locked laser.
The method according to the present invention may further comprise the step of supplying said clock pulse to an optical filter to stretch the pulse width of said clock pulse.
The signal light to be supplied to the first waveform shaper may be WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths. For example, the WDM signal light is supplied to an optical delay circuit, in which each timing of the plurality of optical signals is changed in the time domain. By a specific function of the first waveform shaper, the WDM signal light is converted into the intermediate signal light given as an optical time division multiplexed signal having a single wavelength. In this case, the clock pulse preferably comprises a plurality of clock pulses respectively corresponding to the plurality of optical signals. As a result, the regenerated signal light becomes WDM signal light composed of a plurality of optical signals having wavelengths respectively corresponding to the wavelengths of the plurality of clock pulses.
As the optical delay circuit, an optical medium (e.g., optical fiber) providing chromatic dispersion may be used.
The method according to the present invention may further comprise the step of making the polarization states of the plurality of optical signals constant. In this case, polarization dependence in each waveform shaper can be avoided.
In accordance with another aspect of the present invention, there is provided a device comprising a first waveform shaper for accepting signal light to output intermediate signal light; an optical branch for dividing said intermediate signal light into first and second signal lights; a clock recovery circuit for accepting said first signal light to output a clock pulse; and a second waveform shaper for accepting said second signal light and said clock pulse to output regenerated signal light synchronous with said clock pulse. By using this device, the method according to the present invention can be easily carried out.
In accordance with a further aspect of the present invention, there is provided a system comprising a first optical fiber transmission line for propagating signal light; an optical regenerator connected to said first optical fiber transmission line for converting said signal light into regenerated signal light; and a second optical fiber transmission line connected to said optical regenerator for propagating said regenerated signal light. The optical regenerator may be provided by the device according to the present invention. By using this system, the method according to the present invention can be easily carried out.
In accordance with a still further aspect of the present invention, there is provided a method comprising the steps of converting WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths into an optical time division multiplexed signal; obtaining a clock pulse having a frequency corresponding to the speed of each of said plurality of optical signals; and supplying said optical time division multiplexed signal and said clock pulse to a waveform shaper to obtain regenerated signal light.
Preferably, said step of obtaining said clock pulse comprises the step of regenerating a plurality of clock pulses respectively corresponding to said plurality of optical signals.
This method may further comprise the step of sequentially delaying said plurality of clock pulses by a predetermined delay time.
The step of regenerating said plurality of clock pulses may comprise the step of supplying said optical time division multiplexed signal to a plurality of clock recovery circuits.
Alternatively, the step of regenerating said plurality of clock pulse may comprise the steps of supplying said WDM signal light to a plurality of optical filters having pass bands respectively corresponding to the wavelengths of said plurality of optical signals, and supplying outputs from said optical filters to a plurality of clock recovery circuits, respectively.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.